Powder metal manufacturing is a known process. Fine particles of metal powder are compacted to the desired shape and subsequently sintered in a furnace. Sintering causes melting and welding of powder particles on their respective surfaces to form a part.
There are two inherent disadvantages to powder metal process. The first problem is that it is very difficult, if not impossible, to compact the powder metal to 100% density of the material to eliminate all voids and porosities. Voids reduce the strength of the part and can also create internal corrosion. The solution to this problem is impregnation, or infiltration, of the voids by copper, resin, or other material. Resin impregnation has limited applications because it does not increase material properties and the resin melts down at low temperatures, for example, in a painting oven. Copper impregnation, while adding to the material strength, is costly and more importantly creates dimensional changes to the part reducing the dimensional accuracy.
The second problem with powder metal process is that the capability of compacting the powder in the horizontal direction, perpendicular to the compaction force, is very limited. Most powder metal operations use gravity to fill the powder in the mold or die. The compaction direction is vertical. This means that current products like multiple ribbed pulley grooves cannot be made by a powder metal/sinter process, while sprocket teeth can be manufactured using a powder metal/sinter process.
Densification of powder metal (PM) is a technology that has been around as long as the PM technology itself. There are many known methods to accomplish densification. The most common process is cold forging. Hot forging and rolling are also known for densification.
Spinning is also well known as a metal forming process. Spinning is generally defined as when a work piece is turned and the tool, which is also usually able to turn i.e. installed on bearings, forms the workpiece. The most common spinning process changes the shape of a sheet metal blank while keeping it in sheet form. Examples of this are forming pots and pans, spinning grooved pulleys, as well as spinning the front curved shape of jet engines (a hard to form titanium alloy).
In general spinning has the ability to flow metal much more readily than other processes. As the part turns, the roller brings the metal to its plastic state, flows it, and departs it. Every element of the material is brought to plastic flow over and over again as the part turns. Consequently, spinning can flow the metal thousands of percent, virtually without limit.
Flow forming by spinning is done in the same manner as for spinning, but usually under greater loads and pressures. The material is first brought into a plastic state, at which point it is flowed like pottery clay. Examples of this type of spinning are making multiple-grooved pulleys from flat blanks, spinning hubs from flat blanks, and spinning gears from flat blanks.
On the other hand, press forming can do the same, but only through use of multiple stations, perhaps thousands of stations or more. Spinning can flow metal on a single piece of equipment as opposed to a press which for a given part might require thousands of stations, each incrementally forming the part per cycle.
Automotive pulleys are conventionally made by spinning sheet metal using a number of different processes known in the art. However, for crankshaft damper pulleys, in many cases the mass of the pulley has to be higher than what spun sheet metal pulleys can provide. A higher mass is required to provide the proper amount of inertia necessary to damp crankshaft vibration.
Conventionally, a higher mass is achieved by using cast iron pulleys instead of spun sheet metal. The problem with cast iron is that due to its manufacturing process, i.e. cast in sand, it has to be machined to achieve the desired final shape and dimension. Machining is a relatively expensive operation as it adds time and labor costs as well as creating waste. Furthermore, machined grooves of a multiple ribbed or other types of pulleys are rougher than a spun part due to the existence of machining marks (grooves) caused by the cutting head. This leads to decreased belt life.
Furthermore, machining exposes porosities, which are inherent in castings. The sharp edges of open porosities are detrimental to a belt running on the grooves. Machining the grooves also cuts through the grain structure creating an overall weaker structure.
Representative of the art is U.S. Pat. No. 3,874,049 (1975) to Ferguson which discloses a method of forming powdered metal parts wherein a sintered preform is cold formed and during such forming shear forces are applied to the surface of the preform where a bearing surface is desired by causing a moveable die to penetrate and wipe along such surface area of the preform.
Also representative of the art is U.S. Pat. No. 5,947,853 (1999) to Hodjat et al. which discloses a pulley with an integral hub spun-roll formed of a disc of sheet metal where the hub has a thickness greater than the thickness of the sheet metal.
What is needed is a method of flow forming sintered or cast metal parts. What is needed is a method of densifying sintered powder metal parts by spinning. What is needed is a method of manufacturing low cost, net-shaped, high inertia pulleys for crankshaft dampers by flow forming sintered or cast metal parts. The present invention meets these needs.